Annex C ADSL operates over twisted pairs that are bundled with Japanese ISDN-carrying twisted pairs. FIG. 1 is a plot depicting TCM (Time Compression Multiplex)-ISDN crosstalk noise. The crosstalk noise 101 generated by the ISDN twisted pairs is time varying, and is synchronous to a clock, called a TTR clock, having a time period 102. The TTR clock is an example of a noise clock.
Unlike an Annex C ADSL system, an Annex A ADSL system has no means of synchronization to a TTR clock and, indeed, cannot handle a time varying noise source. Thus all computations in an Annex A ADSL system such as the computations used for the derivation of equalizers (gain adjust, bit allocation, etc.) are based on a continuous average measure of the noise environment.
By contrast, an Annex C ADSL system implements two different receivers, each optimized to the characteristics of its associated noise phase. While Annex C ADSL adapts better to time varying noise than Annex A ASDL, a draw-back with Annex C ASDL is that the instantaneous bit rate varies in each phase. In order to achieve a constant bit rate, significant buffering is needed, which causes latency added to the data transmission. In some applications, the additional latency is unacceptable. Thus a fallback position is to use an Annex A ASDL solution, where disadvantages in handling time varying noise are to be expected.